1. Field of the Invention
The present invention relates to a strand guiding apparatus installed in continuous casting equipment.
2. Description of the Background Art
In continuous casting equipment, a metal strand is allowed to solidify while being withdrawn downward from a bottom of a mold using a strand guiding apparatus, thereby producing cast pieces such as billets and slabs. Many patent documents refer to strand guiding apparatuses.
For example, Japanese Examined Patent Publication No. H02-32062 discloses a strand guiding apparatus including a plurality of pairs of drive rollers along a moving direction of a strand. Each pair of drive rollers is provided to sandwich the strand to drivingly guide the strand. The driving force from a drive motor is transmitted to each drive roller via a speed reducer. In the strand guiding apparatus of this patent document, a worm reducer is used as the speed reducer.
There are roughly three reasons why such worm reducers are used. The first reason relates to the installation space limitation. Specifically, multi-strand continuous casting is carried out in continuous casting equipment of recent years in view of the productiveness, and casting is carried out in multi-strand continuous casting equipment with strand guiding apparatuses arranged in a horizontal direction. In other words, strand guiding apparatuses are arranged on the opposite sides of a strand guiding apparatus. Accordingly, enough installation spaces for drive motors and the like cannot be obtained at lateral sides in many cases. However, worm reducers, which can transmit a driving force in an orthogonal direction, can eliminate the likelihood that a positional interference occurs between drive motors of one strand guiding apparatus and those of another strand guiding apparatus.
The second reason relates to the installation space of a worm reducer. Specifically, in the worm reducer, a large speed reduction ratio can be accomplished simply by reducing the lead angle of a worm without changing the outer dimensions of the speed reducer. Since a large speed reduction ratio can be accomplished by a worm reducer, high performance is not required for a primary speed reducer, which thus makes it possible to employ a smaller-sized primary speed reducer and a universal joint. Therefore, worm reducers capable of giving a large speed reduction ratio by a compact mechanism without taking up a large installation space are suitably used in multi-strand continuous casting equipment having a limited installation space.
The third reason relates to the prevention of strand drop. Specifically, a worm reducer having a large gear ratio of 1/40 to 1/60 will cause self-locking which is specific to a drive mechanism using a worm. In the state where this self-locking works, the drive roller cannot rotate the drive motor while the drive motor can rotate the drive roller. In other words, even if the drive motor stops for a certain reason, e.g., power stoppage, the drive rollers are locked in a stopped state when the worm reducer causes self-locking, therefore there is no likelihood that the strand drops. Thus, worm reducers whose gear ratios are conventionally set at a large value of 1/40 to 1/60 are preferably used particularly in vertical continuous casting equipment which is likely to receive the weight of strand.
As described above, worm reducers used in continuous casting equipment are normally set at such a large gear ratio that self-locking is likely to work to prevent the strand from dropping under undesired conditions, e.g., power stoppage. However, large weight of the strand or a dummy bar acts on drive rollers in continuous casting equipment, particularly in vertical continuous casting equipment during a start operation using the dummy bar and during a casting operation, which consequently causes drive rollers to rotate, i.e., “reverse-powering”, owing to a friction force generated on contact surfaces of the drive roller and the strand due to a falling force exerted from the strand or the dummy bar.
However, in the case where the worm reducer is set at such a large gear ratio as to cause self-locking as described above, no load is transmitted from the drive roller to the drive motor. Thus, the drive motor does not sense that the drive roller is in the “reverse-powering” condition, and it becomes difficult to control the output of the drive motor based on the load sensed by the drive motor. As a result, in the case of holding or withdrawing the strand or the dummy bar using a plurality of drive motors, the drive motor continues to generate driving forces even for the case that outputs of the drive roller should be reduced, thereby leading to the problem that excessive loads damage the worm reducer.
It is possible to control a plurality of drive rollers to have a predetermined rotational speed regardless of loads on the drive motor. However, the respective diameters of the drive rollers slightly vary due to manufacturing errors and friction. Thus, the respective circumferential speeds of the drive rollers are not strictly equal to one another even if the rotational speeds are the same. Further, the moving speed of the strand changes by as much as a pressure control at a lower portion of the strand guiding apparatus, particularly at a lower portion of a strand pressure controller. Therefore, it is also difficult to accurately conform the circumferential speeds of all the drive rollers to the moving speed of the strand.
More specifically, even if it is tried to control the rotational speed of the drive rollers, there is a possibility that such a control leads to a situation where the circumferential speed of a certain drive roller is faster than the moving speed of the strand, but that of another drive roller is slower, i.e., the circumferential speed differs among the drive rollers. In the case where the strand is guided by the drive rollers whose circumferential speeds are different, there may be a problem that slippage occurs between the strand and surfaces of the drive rollers to lower the surface quality of the strand, and a problem that a force acts in a tensile direction or compression direction to cause “chatter” of the strand. Of course, large loads transmitted to the worm reducer, e.g., teeth of the worm and the worm wheel, will may break them. This is the big problem.